Flagpole J for 10 Meters
 

Jimmie D wrote:
"The book shows a .25wl of antenna and a .25wl of stub,---."

The J antenna is often described as a half-wave of antenna with a
quarter-wave matching stub.

Function of the stub is to shunt the antenna at a point so that the
parallel combination of antenna and stub equal Zo.

If the antenna were only 1/4-wavelength, reflection from its
open-circuit end at its drive point may yet be compensated by a .25wl
stub.

At the 1/4-wavelength stub`s short-circuit, phase of the reflected
voltage is reversed. Voltage reflected from the open-circuit tip of the
antenna is not reversed in phase by its reflection. Both waves travel
the same distance to the point where they meet at the antenna drive
point. These tend to offset each other.

Terman describes impedance matching using stubs on page 109 of his 1955
opus:
"Thus, although a reflected wave is present in the length l1 because of
reflection from ZL, there is no reflected wave on the generator side of
the stub line as a result of the cancellation of the two reflected
waves."

Best regards, Richard Harrison, KB5WZI

Flagpole J for 10 Meters
 

Richard Harrison wrote:
"Thus, although a reflected wave is present in the length l1 because of
reflection from ZL, there is no reflected wave on the generator side of
the stub line as a result of the cancellation of the two reflected
waves."

What? "Cancellation of the two reflected waves"???
Some of the gurus here say that cannot happen,
that there is never any interaction between waves.
--
73, Cecil http://www.w5dxp.com

Flagpole J for 10 Meters
 

On Mon, 18 Feb 2008 13:33:44 -0500, "Jimmie D"
wrote:

To my surprise the 50 ohm
point where you connected the feedline to stub was in the same place on both
antennas.

Hi Jimmie,

That would make sense given the matching section is to match a 50 Ohms
drive to any load.

Initial assumption was this was some kind of screw up. I guess
not. This AM I spent some time building the short J out of some galvanize
pipe and it works as advertise

If you are still talking about the curiosity of a quarterwave match to
a quarterwave radiator, then it should work as advertised. The
curiosity factor is like wondering why anyone would buy a car with
five wheels when the world has been traveling on four for millennia.

73's
Richard Clark, KB7QHC

Flagpole J for 10 Meters
 

Cecil, W5DXP wrote:
"What ? "Cancellation of the two reflected waves"??? Some of the gurus
here say that cannot happen, that there is never any interaction between
waves."

I was quoting Terman word for word. In more than 60 years of studying
him I have never found him mistaken. Terman uses words like
cancellation, waves, and power. These words make me comfortable even
when I don`t understand what was said upon first reading.

On the subject of the Flagpole J, Kraus has some words on matching with
a short-circuited 1/4-wave stub on pages 719 and 720 of his 3rd edition.
These reminded me of a conversation with Charlie Wright of A.D. Ring &
Associates I had over 50 years ago. Although Kraus had already
published, we didn`t have his book.

Kraus writes:
"A modification of this antenna is shown at (e) in which a
short-circuited 1/4-wave section of coaxial line is connected in
parallel with the antenna terminals. This widens the impedance bandwidth
and also places the stub antenna at ground potential. This is desirable
to protect the transmission line from lightning surges."

From experience, Kraus has proved to be correct and you can bank on what
he says. Frequency agile amateurs find resonant lengths of transmission
line troublesome at times, but more bandwidth over a narrow range and
protection from lightning are big assets where needed.

Best regards, Richard Harrison, KB5WZI

Flagpole J for 10 Meters
 

Richard Harrison wrote:
"A modification of this antenna is shown at (e) in which a
short-circuited 1/4-wave section of coaxial line is connected in
parallel with the antenna terminals. This widens the impedance bandwidth
and also places the stub antenna at ground potential. This is desirable
to protect the transmission line from lightning surges."

All the advantages of a Double Bazooka using
only 1/4WL of coax.
--
73, Cecil http://www.w5dxp.com